Ice on aircraft wings or other aircraft surfaces is a serious problem because it disturbs the laminar airflow across these surfaces and could result in loss of aerodynamic lift. Likewise, the presence of ice on roads is a serious problem because, as is well known, icy road conditions can cause vehicular accidents which could result in serious injury or death to the vehicle operator or others.
There are numerous systems known in the art to detect ice on a surface such as, for example, an aircraft surface or a road surface. Ice detectors for aircraft are disclosed in U.S. Pat. No. 2,359,787 issued to M. F. Peters, et al. on Oct. 10, 1944 for "Ice Detector"; U.S. Pat. No. 4,054,255, issued to Bertram Magenheim on Oct. 18, 1977; U.S. Pat. No. 4,604,612, issued to Watkins, et al. on Aug. 5, 1986 for "Ice Detector"; U.S. Pat. No. 4,775,118 issued to James W. Daniels on Oct. 4, 1988 for "Ice Detecting System"; and U.S. Pat. No. 5,180,122 issued to Donald J. Christian, et al. on Jan. 19, 1993 for "Apparatus For Deicing".
Some road surface ice detector systems are disclosed in U.S. Pat. No. 4,274,091 issued to Peter W. Decker on Jun. 16, 1981 for Road Surface Ice Detector and Method for Vehicles; and, U.S. Pat. No. 4,690,553 issued to Hiroski Fukamiza, et al. on Sep. 1, 1987 for "Road Surface Condition Detection System".
None of the above identified patents disclose the use of an imaging device (camera) in combination with polarizer means to detect the presence and thickness of ice on a surface.
The present invention is directed to both passive and active ice detection systems for monitoring and detecting the presence and thickness of ice on a surface.
The ice detection system of the present invention measures the polarization changes introduced by the polarizing characteristics of the hexagonal crystals of ice. The polarization may be linear, elliptical, depolarized or circular. As is well known, there are several sources of polarized light in a polarization image of a surface which contains ice. The strongest source of polarization lies in the fact that ice emits radiation according to Fresnel's law. Fresnel's law describes the quantity of emitted polarized light in the planes parallel and perpendicular to the plane of emission as a function of the angle of emission. The angle of emission is defined as the angle between the normal to the surface and the angle at which the surface is being viewed. The plane of emission is defined as the plane containing the normal of the surface and the angle of emission. The difference in the quantity of light emitted in the planes parallel and perpendicular to the plane of emission results in linearly polarized, emitted light.
A second source of polarization in ice lies in the fact that ice is a naturally occurring birefringent crystal. Birefringence is a material property that selectively changes the speed of light depending upon its state of polarization. A retardation or phase change results from the different speeds of light for light composed of particular polarization states propagating in ice. The phase change transforms light of one polarization state into another, from linear to elliptical for example. Birefringence is also introduced in ice crystals by stress and strain in the formation of the ice.
A third source of polarization lies in the multiple reflections and refractions that light undergoes in leaving the ice crystal. A source of depolarization lies in the air bubbles, impurities, and grain boundaries of polycrystalline ice. Each of these sources of polarization gives rise to the distinctive polarization signature of ice that differentiates it from the underlying surface.
The present invention exploits these facts and provides both a passive and active ice measurement system each of which can use any of several types of cameras, which facilitate measurement of the emittance or reflection of radiation from the ice and other surrounding surfaces. The key to ice detection system is the ability to very precisely measure by means of a polarization device, the polarization changes introduced on the background signature by the presence of ice. The basic system is comprised of a state-of-the-art camera system with integrated polarization means.
Both the active and passive system use a camera designed to collect images of the scene or area of interest, a polarization device that varies these images while images are collected in a memory device, a processor to process the collected images, and a display to display the processed images to the operator. The camera also includes a lens and a sensitive focal plane array (FPA). Different lens configurations as well as zoom lenses are required dependent upon the application. The polarization device (such as a linear polarizer) is placed behind the camera lens and in front of the focal plane array (FPA).
The polarization device can take the form of a linear polarizer, which as stated hereinbelow, may be one of many forms such as polaroid material, a wire grid, Wollaston prism, or Brewster plates.
As stated hereinbelow an electrically controllable retarder such as liquid crystal variable retarder may also be used. The retardation device may also be rotated.
The polarization device provides the means by which the polarization state of the light incident on the FPA is determined. Depending on the elements of the polarizing device, one or all polarization states can be characterized. One common embodiment of the polarizing device is a rotating linear polarizer.
The polarization device introduces a modulation of the background measurement whose amplitude is related to the amount of polarized radiation emitted or reflected by the ice. The polarization device can be varied in a step wise fashion or varied in a continuous fashion that is synchronized with the frame rate of the camera to produce a modulated signal. For example, if the polarizer is a linear polarizer rotating at a frequency of 3 revolutions per second (RPS) a camera with a 30 Hertz frame rate would produce a 3 Hertz signal based upon the various amplitude measurements made at 36 degree increments. The rotation of the polarizer creates an amplitude modulation in the areas of the image where the emitted radiation is polarized.
It is, therefore, an object of the present invention to provide a system for detecting ice on a surface.
It is another object of the present invention to provide such a system which is a passive real-time ice detection system.
It is still another object of the present invention to provide such a system which is an active ice measurement system.
It is yet another object of the present invention to provide such an ice detection system wherein imaging in combination with a polarizer, is used to not only detect ice on the surface but to provide actual indications of the thickness of the ice.
These and other objects of the present invention will become more readily apparent from the following description and drawing.